Hydraulic system for controlling a work implement

ABSTRACT

A hydraulic system includes a cylinder assembly with a rod, a pressurized fluid source, a fluid tank, and a control valve. The control valve is fluidly connected to the cylinder assembly. The hydraulic system also includes a line relief, first makeup valve, and second makeup valve fluidly connected to a head end of the cylinder assembly. The second makeup valve is configured to provide fluid flow to the head end of the cylinder assembly when the rod is urged toward an extended position and the control valve is in a neutral position.

TECHNICAL FIELD

The present disclosure relates generally to a hydraulic system for controlling a work implement. Specifically, the disclosure relates to a hydraulic system and method of eliminating boom cylinder head end void and/or for improving excavator efficiency.

BACKGROUND

Machines with work implement systems actuated with hydraulic circuits and hydraulic cylinder assemblies sometimes include at least one boom cylinder, a stick cylinder, and a bucket cylinder. During some dig applications, an operator gives “full bucket close” and “stick in” commands at the same time. Consequently, the hydraulic cylinders are operable such that the bucket is urged to a fully closed position and the stick is urged in. In such situations, the bucket may interact with the soil, causing a moment on the linkage between the boom, stick, and bucket. As a result of the moment, the linkage tries to extend the boom cylinder(s) while the boom cylinder is being operably maintained in a neutral position. Any resultant movement of the boom cylinder while being controlled in a neutral operating position creates a void (e.g., entrained air in oil) in the boom cylinder head end because the hydraulic circuit associated with the boom cylinder cannot provide enough fluid flow to fill the boom cylinder head end.

United States Patent Application Publication US 20110175005 A1 discloses a hydraulic system for a machine. The hydraulic system includes a dump cylinder including a rod, a head end, and a rod end. The system includes a controllable selector valve for controlling oil pressure to the head end and the rod end of the dump cylinder. The hydraulic system also includes an anti-void release valve provided in the oil path of the dump cylinder.

SUMMARY OF THE INVENTION

In one aspect, the disclosure includes a hydraulic system including a hydraulic cylinder assembly, a pressurized fluid source, a fluid tank, a metering control valve, a line relief and first makeup valve, and a second makeup valve. The hydraulic cylinder assembly includes a cylinder, a rod, a head end including a head pressure, and a rod end including a rod pressure. The metering control valve includes a rod extension position, a rod retraction position, and a neutral position. The metering control valve is fluidly connected to the head end, the rod end, the fluid source, and the fluid tank, and includes a valve opening configured to direct fluid from the pressurized fluid source when the metering control valve is in the rod extension position. The valve opening has opening area less than about 3% of a maximum opening area of the valve opening over about the first 50% of full spool displacement. The line relief and first makeup valve are fluidly connected to the head end of the cylinder assembly. The second makeup valve is fluidly connected to the head end of the cylinder assembly, and is configured to provide fluid flow to the head end of the cylinder assembly when the rod is urged toward an extended position at a time when the metering control valve is in the neutral position.

In another aspect, the disclosure includes a machine including a power source, a work implement, and a hydraulic system. The hydraulic system includes a hydraulic cylinder assembly including a cylinder, a rod, a head end including a head pressure, and a rod end including a rod pressure. The hydraulic system further includes a fluid source, a fluid tank, and a metering control valve including a rod extension position, a rod retraction position, and a neutral position. The control valve is fluidly connected to the head end, the rod end, the fluid source, and the fluid tank, and includes a valve opening configured to direct fluid from the pressurized fluid source when the metering control valve is in the rod extension position. The valve opening is less than about 10 mm² over about the first 6.5 mm of spool displacement. The hydraulic system also includes a line relief and makeup valve fluidly connected to the head end of the cylinder assembly, and a second makeup valve fluidly connected to the head end of the cylinder assembly. The second makeup valve is configured to provide an additional fluid flow to the first makeup valve fluid flow to the head end of the cylinder assembly when the rod is urged toward an extended position at a time when the metering control valve is in the neutral position or close to the neutral position.

In another aspect, the disclosure includes a method for operating a machine that includes a bucket, a stick, and a boom. The method includes selectively operating a boom metering control valve in a neutral position or close to the neutral position to maintain the boom in desired position and selectively operating a stick and a bucket of the machine to close the bucket and retract the stick while the boom metering control valve is in the neutral position or close to the neutral position. The method further includes, at times when said selective operation of the stick and bucket causes extension of the boom while the boom metering control valve is in the neutral position, providing fluid flow to the head end of the boom cylinder assembly via a first makeup valve fluidly connected to the head end of the cylinder assembly and a second makeup valve fluidly connected to the head end of the cylinder assembly. The method also includes selectively operating the metering control valve in a rod extension position to raise the boom to a desired position. The boom metering control valve includes a valve opening configured to direct fluid from the pressurized fluid source when the metering control valve is in the rod extension position, wherein the valve opening is less than about 10 mm² over about the first 6.5 mm of spool displacement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary embodiment of a machine.

FIG. 2 illustrates an exemplary first embodiment of a hydraulic system with a metering control valve in a neutral, closed position.

FIG. 3 illustrates the exemplary first embodiment of the hydraulic system with the metering control valve in a rod extension position.

FIG. 4 illustrates the exemplary first embodiment of the hydraulic system with the metering control valve in a rod retraction position.

FIGS. 5A and 5B illustrate boom head end fluid and total volumes, and also volumes of entrained air versus time for a conventional hydraulic system and a hydraulic system in accordance with exemplary aspects of the disclosure.

FIG. 6 illustrates an exemplary valve opening area versus spool displacement relationship for a boom cylinder metering valve in accordance with aspects of the disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to specific embodiments or features, examples of which are illustrated in the accompanying drawings. Generally, corresponding or similar reference numbers will be used, when possible, throughout the drawings to refer to the same or corresponding parts.

Referring now to FIG. 1, an exemplary embodiment of machine 100 is illustrated. In the embodiment illustrated, the machine 100 is depicted as a vehicle 104, and in particular an excavator 106. In other embodiments, the machine 100 may include any system or device for doing work with a hydraulically powered work implement control system 108 which would be known to an ordinary person skilled in the art.

The vehicle 104 may include but is not limited to vehicles that perform some type of operation associated with a particular industry such as mining, construction, farming, transportation, etc. and operate between or within work environments (e.g. construction site, mine site, power plants, on-highway applications, marine applications, etc.). Non-limiting examples of vehicle 104 include cranes, earthmoving vehicles, mining vehicles, backhoes, loaders, material handling equipment, and farming equipment.

Machine 100 is equipped with systems that facilitate the operation of the machine 100 at worksite 110. In the depicted embodiment, these systems include the work implement control system 108, a drive system 112, and a power system 114 that provides power to the work implement control system 108 and the drive system 112. In the depicted embodiment, the power system 114 includes an engine 136, for example an internal combustion engine. In alternative embodiments the power system 114 may include other power sources such as electric motors (not shown), fuel cells (not shown), batteries (not shown), ultra-capacitors (not shown), electric generators (not shown), and/or any power source which would be known by an ordinary person skilled in the art.

The drive system 112 may include a transmission (not shown), and ground engaging devices 115. The transmission may include any device or group of devices that may transfer force between the power system 114 and the ground engaging devices 115. The transmission may include one or more of a mechanical transmission, variator, gearing, belts, pulleys, discs, chains, pumps, motors, clutches, brakes, torque converters, fluid couplings and any transmission which would be known by an ordinary person skilled in the art.

The work implement control system 108 includes a work implement 116, which may perform work at worksite 110. In the depicted embodiment, the work implement 116 is a bucket 126. In alternative embodiments the work implement may include other types of work implements 116 such as (but not limited to) blades, lift groups, material handling arms, multi-processors, rakes, shears, snow plows and snow wings.

The work implement control system 108 may include any members, and linkages; as well as any systems and controls to actuate the members and linkages as a function of operator, autonomous system, or other inputs, to maneuver the work implement 116 to perform work at worksite 110, which would be known by an ordinary person skilled in the art.

In the depicted embodiment of a excavator 106, the work implement control system 108 includes a boom 122, a stick 124, the bucket 126, at least one boom cylinder assembly 128, a stick cylinder assembly 130, a work implement cylinder assembly 102, a work implement linkage 134, a controller 182, and an operator interface 188. The boom cylinder assembly 128 includes a boom cylinder 133 and a boom rod 132.

In the depicted embodiment, machine 100 includes a cab 118 including the operator interface 188. The operator interface 188 may include devices with which an operator communicates with, interacts with, or controls the machine 100. In one embodiment, the operator interface 188 may include devices with which the operator interacts physically. In another embodiment, the devices may operate with voice activation. In still other embodiments, the operator may interact with the operator interface 188 in any way a person skilled in the art would contemplate. In the depicted embodiment, the operator interface includes a joystick 120.

The operator interface 188 may be operable to generate commands to the work implement control system 108 to move the work implement 116 to perform work at the worksite 110. The operator interface 188 may be operable to generate work implement control system 108 control commands as a function of predetermined movement from an operator. In alternative embodiments, machine controls encoded in the controller 182 onboard the machine 100, or an autonomous control system located remotely from the machine 100 may communicate work implement control system 108 commands.

In the depicted embodiment, an operator may enter commands to maneuver the work implement 116 through moving the joystick 120. These commands may be transmitted via sensors and communication links to the controller 182. The controller 182 may transmit signals via communication links to actuate hydraulic fluid valves to allow pressurized fluid flow to and from the cylinder assemblies 128, 130, 102 as is well known in the art. As pressurized fluid flows to and from the cylinder assemblies 128, 130, 102, rods (such as boom rod 132) may extend from and retract into cylinders (such as boom cylinder 133) to move the work implement 116. In other embodiments hydro-mechanical control systems may transmit operator commands to actuate the work implement 116.

In the depicted embodiment, a work implement linkage assembly 134 is operably connected to the work implement cylinder assembly 102 and the work implement 116 to actuate work implement 116 in a desired way.

The controller 182 may include a processor (not shown) and a memory component (not shown). The processor may include microprocessors or other processors as known in the art. In some embodiments the processor may include multiple processors. The processor may execute instructions transmitted through the operator interface 188 or other means such as remote or autonomous controls to perform work at the worksite 110 with work implement 116. The memory component may include any form of computer-readable media which would be known to an ordinary person skilled in the art now or in the future. The memory component may include multiple memory components.

The controller 182 may be enclosed in a single housing. In alternative embodiments, the controller 182 may include a plurality of components operably connected and enclosed in a plurality of housings. The controller 182 may be located on-board the machine, or may be located off-board or remotely.

The controller 182 may be communicatively connected to the operator interface 188 to receive operator command signals, and operatively connected to hydraulic valves to control movement of the work implement 116. The controller 182 may be communicatively connected to one or more sensors or other devices to receive signals indicative of machine 100 system operating parameters.

An operator, or an autonomous function, may desire to dig earth or other material at work site 110 with the depicted excavator 106, and then dump the material into a haul truck (not shown) or other holding vehicle. As the work implement control system 108 responds to dig commands, for example, “stick in” and “bucket close,” the stick cylinder assembly 130 may extend so that the stick 124 is urged in toward the cab 118, and the work implement cylinder assembly 102 may extend so that the bucket 126 may begin to close, moving downwards and curling inward towards the stick 124 and cab 118, digging material and then holding it as is well known by ordinary persons skilled in the art. While the bucket 126 is digging, interaction between the bucket 126 and the material the bucket 126 is digging may cause a resistive load to be applied to the work implement 116. This resistive load may create a moment on the work implement control system 108, which may cause an extension of the boom cylinder assembly 128 even though the operator is not inputting a “boom up” command. An unintended extension of the boom cylinder assembly 128 may create a void in the boom cylinder 133, which requires that the boom cylinder 133 be filled with fluid before the boom cylinder 133 would move in response to a subsequent “boom up” command by the operator. Boom cylinder 133 void is well known by ordinary persons skilled in the art.

The operator, or an autonomous function, may give a command to raise the boom 122 to position the loaded bucket 126 containing the material over the haul truck, and then begin a dump function. During the raise function, the boom cylinder void requires that the boom cylinder 133 be filled with fluid before the boom cylinder 133 would move. Filling the void takes a period of time, for example, at least about 0.5 second, which introduces a delay in the response of the boom cylinder 133 to the operator's “boom up” command. During the dump function the work implement cylinder assembly 102 may be retracted causing the bucket 126 to open, rotating outwards from the stick 124 and cab 118, and dump the material into the haul truck as is well known by ordinary persons skilled in the art.

Referring now to FIGS. 2, 3, and 4, a first embodiment of a hydraulic system 200 is depicted. The system 200 includes a first hydraulic circuit 201 and a second hydraulic circuit 208. The first hydraulic circuit 201 includes a metering control valve 204 and at least one hydraulic cylinder assembly 202 with a head end 212 having a head pressure and a rod end 214 having a rod pressure. Metering control valve 204 includes a closed position, a rod extension position, and a rod retraction position. FIG. 2 depicts system 200 with metering control valve 204 in the closed position and illustrates fluid flow when dig commands of “full bucket close” and “stick in” result in interaction between the bucket 126 and soil and cause extension of the boom rod 132 from the boom cylinder 133. FIG. 3 depicts system 200 with metering control valve 204 in the rod extension position and illustrates fluid flow when the head pressure exceeds the rod pressure (actual pressure value is load dependent) during most of the boom up operation. During an initial portion of the dig cycle, the rod end pressure may exceed the head end pressure (actual pressure value is load dependent) at the neutral boom lever position due to bucket and soil interaction. FIG. 4 depicts system 200 with the boom metering control valve 204 in the rod retraction position. Generally when the control valve 204 is in this position, the boom cylinder head end pressure is greater than the rod end pressure due to overrunning loading condition on the boom cylinder. A boom metering control valve and a boom regen valve (not shown) may provide flow to prevent the boom cylinder rod end void as is known in the art.

The hydraulic system 200 is suitable for use in the excavator 106 of FIG. 1. The hydraulic cylinder assembly 202 may, for example, correspond to the boom cylinder assembly 128. The hydraulic system 200 may also be suitable for actuating other linkages illustrated in FIG. 1, or for actuating other tools on other machines 100.

The first hydraulic circuit 201 includes a hydraulic cylinder assembly 202, a pressurized fluid source 206, a fluid tank 210, a boom load control valve 224, and a metering control valve 204. The cylinder assembly 202 includes a head end 212 having a head pressure, a rod end 214 having a rod pressure, a cylinder 290, and a rod 292. The metering control valve 204 includes a closed position (shown in relation to FIG. 2) a rod extension position (shown in relation to FIG. 3) and a rod retraction position (shown in relation to FIG. 4). The metering control valve 204 is fluidly connected to the head end 212, the rod end 214, the fluid source 206, and the fluid tank 210.

The cylinder assembly 202 may include any mechanical actuator operable to apply a substantially unidirectional force through a unidirectional stroke which would be known to an ordinary person skilled in the art. The rod 292 may include a piston which divides the cylinder 290 into two (2) chambers, one on the head end 212, and one on the rod end 214. Each chamber may include a port through which fluid may flow in and out of the chamber. The rod 292 may move back and forth in the cylinder 290 as fluid flows in and out of the chambers, as is known by ordinary persons skilled in the art. The rod 292 may be operably connected to the boom 122.

In the excavator 106 embodiment depicted in FIG. 1, the rod 292 may correspond to boom rod 132 and be operably connected to the boom cylinder assembly 128, to raise and lower the boom 122. Pressurized fluid may flow into the head end 212, extending the rod 292 from the cylinder 290, and raising the boom 122. As pressurized fluid flows into the head end 212, fluid flows out of the rod end 214. Pressurized fluid may also flow into the rod end 214, retracting the rod 292 into the cylinder 290, and lowering the boom 122. As pressurized fluid flows into the rod end 214, fluid flows out of the head end 212.

The fluid source 206 may include any source of pressurized hydraulic fluid which would be known by an ordinary person skilled in the art. The fluid source 206 may include, but is not limited to, a fixed displacement pump (not shown), a variable displacement pump (not shown), a hydraulic fluid accumulator (not shown), or any other pressurized fluid energy storage device. In the depicted embodiment, engine 136 may drive fluid source 206 through one or more gears. In alternative embodiments, the fluid source 206 may include a pump driven in any manner which would be known by an ordinary person skilled in the art. Non-limiting examples include gear driven, belt driven, or electric motor driven pumps. The fluid tank 210 may include any reservoir for holding fluid which would be known by an ordinary person skilled in the art.

Springs hold the metering control valve 204, as shown in FIG. 2, in a position substantially inhibiting flow through the metering control valve 204. One of the springs may have a spring constant of K1, and another spring may have a spring constant of K2. In the depicted embodiment, the metering control valve 204 is hydraulically actuated. In alternative embodiments, metering valve 204 may be solenoid actuated or actuated in anyway known in the art.

The metering control valve 204 may include a rod extension pilot port 238 selectively fluidly connected to a pilot pressurized fluid source 276 through pilot fluid conduit 260. When fluid from the pilot fluid source 276, with a high enough pressure to overcome the force of the K2 spring, flows through pilot fluid conduit 260 to the rod extension pilot port 238, the metering control valve 204 may move to the rod extension position as shown in FIG. 3. The rod extension pilot port 238 may be selectively fluidly connected to the pilot fluid source 276 through a valve (not shown) actuated by a signal from the controller 182, or by other electrical, mechanical, pneumatic, or hydraulic means which would be known by an ordinary person skilled in the art. For example, a solenoid actuated directional valve may be used, which may be actuated by a current signal from the controller 182. The controller 182 may generate this signal as a function of an operator input through operator interface 188, or may generate this signal as a function of an input from a remote or automated control system.

The metering control valve 204 may include a rod retraction pilot port 240 selectively fluidly connected to a pilot pressurized fluid source 276 through pilot fluid conduit 280. When fluid from the pilot fluid source 276, with a high enough pressure to overcome the force of the K1 spring, flows through pilot fluid conduit 280 to the rod retraction pilot port 240, the metering control valve 204 may move to the rod retraction position as shown in FIG. 4. The rod retraction pilot port 240 may be selectively fluidly connected to the pilot fluid source 276 in a similar way as the rod extension pilot port 238.

The pilot fluid source 276 may include the fluid source 206 or the pilot fluid source 276 may be a separate source. The pilot fluid source 276 may, for example, include a pump driven by the engine 136 through gears. Embodiments of the pilot fluid source may include a fixed displacement pump (not shown), a variable displacement pump (not shown), a hydraulic accumulator, or any other pressurized fluid source which would be known by an ordinary person skilled in the art. The pilot fluid source 276 may be driven or powered by the power system 114 through mechanical linkage, electrically, hydraulically, or by any means which would be known by an ordinary person skilled in the art.

In the depicted embodiment, the head end 212 is fluidly connected to the metering control valve 204 via a boom load control valve 224 and fluid conduits 234, 236. The rod end 214 is fluidly connected to the metering control valve 204 through fluid conduit 228. The fluid source 206 is fluidly connected to the metering control valve 204 through a check valve 220 and fluid conduit 222. The tank 210 is fluidly connected to the metering control valve 204 through a back pressure valve 232 and fluid conduits 230 and 231.

As shown in relation to FIG. 3, when the metering control valve 204 is in the boom up position, pressurized fluid may flow in the first hydraulic circuit 201, from the fluid source 206 through the check valve 220, through fluid conduit 222, through a valve opening 244 of the metering control valve 204, through boom load control valve 224, and through fluid conduit 234 to the head end 212 to raise the boom 122. The metering control valve 204 includes a valve opening 244 that is sized and arranged to provide a flow rate of fluid to the boom cylinder 133 during a “boom up” operation so as to provide smooth functionality. When the metering control valve 204 is in the rod extension position, fluid may flow in the first hydraulic circuit 201, from the rod end 214, through fluid conduit 228, through the metering control valve 204, through fluid conduit 230, through back pressure valve 232, and to the tank 210.

As shown in relation to FIG. 4, when the metering control valve 204 is in the rod retraction position, pressurized fluid may flow in the first hydraulic circuit 201, from the fluid source 206 through the check valve 220, through fluid conduit 222, through the metering control valve 204, and through fluid conduit 228 to the rod end 214 to lower the boom 122. When the metering control valve 204 is in the rod retraction position, fluid may flow in the first hydraulic circuit 201, from the head end 212, through conduit 234, through boom load control valve 224, through conduit 236, through the metering control valve 204, through fluid conduit 231, through backpressure valve 232, and to the tank 210.

The second hydraulic circuit 208 is fluidly connected to the head end 212 and the fluid tank 210. The second hydraulic circuit 208 is configured to fluidly connect the head end 212 to the fluid tank when the head pressure falls below the back pressure. The hydraulic circuit 208 is also operable to fluidly connect the head end 212 to the fluid tank 210 when the head pressure exceeds a predetermined value.

As shown in relation to FIGS. 2-4, the second hydraulic circuit 208 may include a makeup valve 246 and a pressure relief valve 252. According to some aspects, the makeup valve 246 and the pressure relief valve 252 may be embodied in a two-in-one combination line relief with makeup function valve 216. The second hydraulic circuit 208 may include a second makeup valve 218.

According to various aspects, the pressure relief valve 252 may be a spring-biased and normally-closed pressure relief valve, and the makeup valve 246 may be a spring-biased and normally closed check valve. The pressure relief valve 252 is configured to open when the head pressure exceeds a predetermined value which overcomes a spring constant K3 of a spring that urges the pressure relief valve to a closed position. The second makeup valve 218 may be a spring-biased and normally closed check valve.

Referring now to FIG. 6, a graph 600 illustrates an exemplary opening area versus spool displacement for the valve opening 244 of the metering control valve 204 in a rod extension position (FIG. 3). Although units are not illustrated in FIG. 6, the x-axis 602 of the graph 600 may represent spool displacement in mm, while the y-axis 604 of the graph 600 may represent the valve opening area in mm². The graph 600 includes a first curve 606 showing a conventional opening versus displacement for a metering control valve and a second curve 608 showing an exemplary opening versus displacement for metering control valve 204 in accordance with the disclosure.

As shown in FIG. 6, the area of the valve opening 244 varies as the spool is displaced in the metering control valve 204. In one embodiment of the illustrated exemplary graph, the area of the valve opening 244 may vary from 0 mm² at 0 mm spool displacement (i.e., closed) to a maximum valve opening area of about 185 mm² at 11 mm spool displacement (i.e., maximum spool displacement). One embodiment of the second curve 608 may represent a reduced initial opening area up to about 10 mm spool displacement. For example, over about the first 5.5 mm spool displacement (or about 50% of total spool displacement), the valve opening area may be less than 5 mm² (or less than 3% of maximum valve opening area). Over about the first 6.5 mm of spool displacement, the valve opening area may be less than about 10 mm² (or less than 5.5% of maximum valve opening area), which is about one-half the area of the valve opening of the conventional valve at 6.5 mm displacement, as represented by curve 606. This limited valve opening area versus spool displacement during initial valve opening may provide a limited flow to the head end of the boom cylinder during dig when a boom up command is given (FIG. 3), which reduces energy loss from the high pressure bucket circuit to the low pressure boom circuit, and may provide a smoother functionality. The flow required to fill the boom head end to prevent cylinder void is provided by the first and second make up valves 246 and 218, respectively.

Referring now to FIGS. 5A and 5B, graphs illustrate the total volume and fluid volume at the head end 212 of the boom cylinder 133 during a dig operation where “full bucket close” and “stick in” commands are given and cause the boom 122 to extend due to interaction of the bucket 116 with soil while the metering control valve 204 is in the neutral position or close to the neutral position. FIG. 5A shows a graph 500 representing the relationship between boom cylinder head end total volume and fluid volume versus time for a hydraulic system similar to the hydraulic system 200, but without the second makeup valve 218. In one embodiment units (not shown) on the x-axis 502 of the graph 500 may be in seconds, while the y-axis 504 of the graph 500 may show volume in liters. The graph 500 includes a first curve 506 showing total head end volume of the boom cylinder 133 and a second curve 508 showing the volume of fluid in the head end 212 of the boom cylinder 133. FIG. 5A shows a second graph 510 including a curve 516 illustrating the entrained air volume in the head end 212 of the boom cylinder 133. Graph 510 includes x-axis 502, while the units (not shown) on the y-axis 514 of the graph 510 in one embodiment may show volume in liters.

FIG. 5B shows a graph 550 of boom cylinder head end fluid and air volume versus time for the hydraulic system 200 with the second makeup valve 218, as described in connection with FIGS. 2-4, and a reduced initial opening area of the valve 244 versus spool displacement as shown in FIG. 6. An embodiment of the graph 550 may include the x-axis 502 showing time in seconds and the y-axis 504 showing volume in liters. The graph 550 includes a single curve 556 showing total head end volume of the boom cylinder 133 and volume of fluid in the head end 212 of the boom cylinder 133. FIG. 5B shows a second graph 560 including a straight line 566 illustrating the absence of entrained air volume in the head end 212 of the boom cylinder 133. Graph 560 includes x-axis 502 and y-axis 514 may show volume in liters. As shown in FIG. 5B, the second makeup valve 218 provides additional fluid flow to the head end 212 of the boom cylinder 133 when the boom 122 extends due to interaction of the bucket 116 with soil while the metering control valve 204 is in the neutral position or close to the neutral position so as to prevent head end void during the dig operation.

INDUSTRIAL APPLICABILITY

In the excavator 106 of FIG. 1, an operator may command a dig function through the operator interface 188. For example, the operator may move the joystick 120 to actuate the boom 122, stick 124, and bucket 126 to dig material on the worksite 110. In another embodiment, a remote or automated system may command a dig function. The controller 182 may receive the dig function commands and transmit signals to actuate the boom cylinder assembly 128, the stick cylinder assembly 130, and the work implement cylinder assembly 102.

Referring to FIGS. 1-4, the controller 182 may transmit signals to fluidly connect the pilot fluid source 276 to the rod extension pilot port 238 of the metering control valve 204 through fluid conduit 260. The pressure of the pilot fluid at rod extension pilot port 238 may overcome the force of the K2 spring and the metering control valve 204 may move to the rod extension position as shown in FIG. 3.

Further as shown in FIG. 3, pressurized fluid may flow from the fluid source 206, through check valve 220, through fluid conduit 222, through the valve opening 244 of the metering control valve 204, through fluid conduit 236, through boom load control valve 224, through fluid conduit 234, and into the head end 212. The arrows marked “H” illustrate the flow of pressurized fluid to the head end 212.

The pressurized fluid flowing into the head end 212 may push the piston of rod 292 and begin extending the rod 292 from the cylinder 290. As the rod 292 begins extending, fluid may flow out of the rod end 214, through fluid conduit 228, through the metering control valve 204, through fluid conduit 230, and to the tank 210. The arrows marked “R” illustrate the flow of pressurized fluid from the rod end 214.

Referring now to FIG. 4, the controller 182 may transmit signals to fluidly connect the pilot fluid source 276 to the rod retraction pilot port 240 of the metering control valve 204 through fluid conduit 280. The pressure of the pilot fluid at rod retraction pilot port 240 may overcome the force of the K1 spring and the metering control valve 204 may move to the rod retraction position as shown in FIG. 4.

Further as shown in FIG. 4, pressurized fluid may flow from the fluid source 206, through check valve 220, through fluid conduit 222, through the metering control valve 204, through fluid conduit 228, and into the rod end 214. The arrows marked “R” illustrate the flow of pressurized fluid to the rod end 214.

The pressurized fluid flowing into the rod end 214 may push the piston of rod 292 and begin retracting the rod 292 into the cylinder 290. As the rod 292 begins retracting, fluid may flow out of the head end 212, through fluid conduit 228, through boom load control valve 224, through fluid conduit 236, through the metering control valve 204, through fluid conduit 231, and to the tank 210. The arrows marked “H” illustrate the flow of fluid from the head end 212.

An operator, or an autonomous function, may desire to dig earth or other material at work site 110 with the depicted excavator 106, and then dump the material into a haul truck (not shown) or other holding vehicle. As the work implement control system 108 responds to dig commands, for example, “stick in” and “bucket close,” the stick cylinder assembly 130 may extend so that the stick 124 is urged in toward the cab 118, and the work implement cylinder assembly 102 may extend so that the bucket 126 may begin to close, moving downwards and curling inward towards the stick 124 and cab 118, digging material and then holding it as is well known by ordinary persons skilled in the art. While the bucket 126 is digging, interaction between the bucket 126 and the material the bucket 126 is digging may cause a resistive load to be applied to the work implement 116. This resistive load may create a moment on the work implement control system 108, which may cause an extension of the boom cylinder assembly 128 even though the operator is not inputting a “boom up” command. An unintended extension of the boom cylinder assembly 128 may create a void in the boom cylinder 133. The combination line relief with makeup function valve 216 and the second makeup valve 218 of the second hydraulic circuit 208 may be configured to provide fluid flow to the head end 212 of the boom cylinder 133 to fill the void. Thus, the boom cylinder 133 is filled with fluid before a subsequent “boom up” command by the operator and the boom cylinder 133 can move in response to the “boom up” command without delay.

The operator, or an autonomous function, may give a command to raise the boom 122 to position the loaded bucket 126 containing the material over the haul truck, and then begin a dump function. During the raise function, since the boom cylinder void has been filled, the boom cylinder 133 can move without delay. During the dump function the work implement cylinder assembly 102 may be refracted causing the bucket 126 to open, rotating outwards from the stick 124 and cab 118, and dump the material into the haul truck as is well known by ordinary persons skilled in the art.

From the foregoing, it will be appreciated that, although specific embodiments have been described herein for purposes of illustration, various modifications or variations may be made without deviating from the spirit or scope of inventive features claimed herein. Other embodiments will be apparent to those skilled in the art from consideration of the specification and figures and practice of the arrangements disclosed herein. It is intended that the specification and disclosed examples be considered as exemplary only, with a true inventive scope and spirit being indicated by the following claims and their equivalents. 

What is claimed is:
 1. A hydraulic system, comprising: a hydraulic cylinder assembly including a cylinder, a rod, a head end including a head pressure, and a rod end including a rod pressure; a pressurized fluid source; a fluid tank; a metering control valve including a rod extension position, a rod retraction position, and a neutral position, the control valve being fluidly connected to the head end, the rod end, the fluid source, and the fluid tank, the metering control valve including a valve opening configured to direct fluid from the pressurized fluid source when the metering control valve is in the rod extension position, the valve opening having an opening area less than about 3% of a maximum opening area of the valve opening over about the first 50% of full spool displacement; a line relief and first makeup valve fluidly connected to the head end of the cylinder assembly; and a second makeup valve fluidly connected to the head end of the cylinder assembly, the second makeup valve being configured to provide fluid flow to the head end of the cylinder assembly when the rod is urged toward an extended position at a time when the metering control valve is in the neutral position.
 2. The hydraulic system of claim 1, wherein the metering control valve includes a rod extension port selectively fluidly connected to a pilot fluid source, and a rod retraction port selectively fluidly connected to the pilot fluid source.
 3. The hydraulic system of claim 2, wherein the metering control valve moves into the rod extension position when the rod extension port is connected to the pilot fluid source, connecting the pressurized fluid source to the head end of the hydraulic cylinder assembly.
 4. The hydraulic system of claim 2, wherein the metering control valve moves into the rod retraction position when the rod retraction port is connected to the pilot fluid source, connecting the pressurized fluid source to the rod end of the hydraulic cylinder assembly.
 5. The hydraulic system of claim 1, further including: a load control valve fluidly connected between the metering control valve and the head end of the cylinder assembly.
 6. The hydraulic system of claim 1, wherein the second makeup valve is a spring-biased and normally closed check valve.
 7. The hydraulic system of claim 1, further including a combination valve, the combination valve including a line relief, the first makeup valve, a spring-biased and normally-closed pressure relief valve; and a spring-biased and normally closed check valve.
 8. The hydraulic system of claim 1, further including a second hydraulic cylinder assembly including a cylinder, a rod, a head end including a head pressure, and a rod end including a rod pressure, the metering control valve and the line relief and first makeup valve being fluidly connected to the head end and the rod end of the second hydraulic cylinder assembly, and the second makeup valve being fluidly connected to the head end of the second hydraulic cylinder assembly, the second makeup valve being configured to provide fluid flow to the head end of the second hydraulic cylinder assembly when the rod of the second hydraulic cylinder assembly is urged toward an extended position at a time when the metering control valve is in the neutral position.
 9. A machine comprising: a power source; a work implement; and a hydraulic system including a hydraulic cylinder assembly including a cylinder, a rod, a head end including a head pressure, and a rod end including a rod pressure, a fluid source, a fluid tank, a metering control valve including a rod extension position, a rod retraction position, and a neutral position, the control valve being fluidly connected to the head end, the rod end, the fluid source, and the fluid tank, the metering control valve including a valve opening configured to direct fluid from the pressurized fluid source when the metering control valve is in the rod extension position, the valve opening having an opening area less than about 3% of a maximum opening area of the valve opening over about the first 50% of full spool displacement, a line relief and first makeup valve fluidly connected to the head end of the cylinder assembly, and a second makeup valve fluidly connected to the head end of the cylinder assembly, the second makeup valve being configured to provide fluid flow to the head end of the cylinder assembly when the rod is urged toward an extended position at a time when the metering control valve is in the neutral position.
 10. The machine of claim 9, wherein the metering control valve includes a rod extension port selectively fluidly connected to a pilot fluid source, and a rod retraction port selectively fluidly connected to the pilot fluid source.
 11. The machine of claim 10, wherein the metering control valve moves into the rod extension position when the rod extension port is connected to the pilot fluid source, connecting the pressurized fluid source to the head end of the hydraulic cylinder assembly.
 12. The machine of claim 10, wherein the metering control valve moves into the rod retraction position when the rod retraction port is connected to the pilot fluid source, connecting the pressurized fluid source to the rod end of the hydraulic cylinder assembly.
 13. The machine of claim 9, further including: a load control valve fluidly connected between the metering control valve and the head end of the cylinder assembly.
 14. The machine of claim 9, wherein the second makeup valve is a spring-biased and normally closed check valve.
 15. The machine of claim 9, further including a combination valve, the combination valve including a line relief, the first makeup valve, a spring-biased and normally-closed pressure relief valve, and a spring-biased and normally closed check valve.
 16. The machine of claim 9, further including a second hydraulic cylinder assembly including a cylinder, a rod, a head end including a head pressure, and a rod end including a rod pressure, the metering control valve and the line relief and first makeup valve being fluidly connected to the head end and the rod end of the second hydraulic cylinder assembly, and the second makeup valve being fluidly connected to the head end of the second hydraulic cylinder assembly, the second makeup valve being configured to provide fluid flow to the head end of the second hydraulic cylinder assembly when the rod of the second hydraulic cylinder assembly is urged toward an extended position at a time when the metering control valve is in the neutral position.
 17. The machine of claim 9, wherein the work implement includes a bucket selectively operable to open and close, the machine further including: a stick selectively operable for extending and retracting the bucket; and a boom selective operable for raising and lowering the bucket, the hydraulic cylinder assembly controlling operation of the boom.
 18. The machine of claim 17, wherein at times when the bucket is operably closed and the stick is operably retracted, interaction of the bucket with a surface causes extension of the boom while the metering control valve is in the neutral position.
 19. The machine of claim 18, wherein the second makeup valve provides fluid flow to the head end of the cylinder assembly while the metering control valve is in a neutral position.
 20. A method for operating a machine that includes a bucket, a stick, and a boom, the method comprising: selectively operating a metering control valve in a neutral position to maintain the boom in desired position; selectively operating a stick and a bucket of the machine to close the bucket and retract the stick while the metering control valve is in the neutral position; at times when said selective operation of the stick and bucket causes extension of the boom while the metering control valve is in the neutral position, providing fluid flow to the head end of the cylinder assembly via a line relief and first makeup valve fluidly connected to the head end of the cylinder assembly and a second makeup valve fluidly connected to the head end of the cylinder assembly; and selectively operating the metering control valve in a rod extension position to raise the boom to a desired position, the metering control valve including a valve opening configured to direct fluid from the pressurized fluid source when the metering control valve is in the rod extension position, the valve opening having an opening area less than about 3% of a maximum opening area of the valve opening over about the first 50% of full spool displacement. 